1. Field of the Invention
This invention relates generally to electronic devices sensitive to electrostatic discharge (ESD), and more particularly to MEMS devices integrated with ESD protection device.
2. Related Art
Any electronic device is sensitive to electrostatic discharge (ESD). One method of protecting from ESD is to use an ESD protection diode that functions as a bypass. ESD protection devices such as Zener diodes, have been built in parallel with the electronic device to be protected. If the voltage across the protected device becomes too high, the Zener diode will act as a short and prevent current from going through the ESD sensitive electronic device.
Zener diodes can be used to regulate the voltage across a circuit. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point it keeps the voltage at that value. A Zener diode is a diode that permits current in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as the Zener voltage.
Zener diodes contain a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides. A reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 10 V will exhibit a voltage drop of 10 V if the reverse bias voltage applied across it is more than its Zener voltage.
The breakdown voltage can be controlled quite accurately by the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%.
In many MEMS devices, including but not limited to piezoresistive accelerometer and pressure sensor devices, the ESD protection device is not built into the device to be protected. With certain MEMS devices the ESD protection device is added during the manufacturing of the product as a separate device with the MEMS device. This results in two discrete components. Until these two discrete components are connected the MEMS device remains unprotected.
Accordingly, there is a need to integrate those two at the fabrication level.